Description:
<jats:title>Summary</jats:title><jats:p>Cph2 from the cyanobacterium <jats:italic>Synechocystis</jats:italic> sp. PCC 6803 is a hybrid photoreceptor that comprises an N‐terminal module for red/far‐red light reception and a C‐terminal module switching between a blue‐ and a green‐receptive state. This unusual photoreceptor exerts complex, light quality‐dependent control of the motility of <jats:italic>Synechocystis</jats:italic> sp. PCC 6803 cells by inhibiting phototaxis towards blue light. Cph2 perceives blue light by its third GAF domain that bears all characteristics of a cyanobacteriochrome (CBCR) including photoconversion between green‐ and blue‐absorbing states as well as formation of a bilin species simultaneously tethered to two cysteines, C994 and C1022. Upon blue light illumination the CBCR domain activates the subsequent C‐terminal GGDEF domain, which catalyses formation of the second messenger c‐di‐GMP. Accordingly, expression of the CBCR‐GGDEF module in Δ<jats:italic>cph2</jats:italic> mutant cells restores the blue light‐dependent inhibition of motility. Additional expression of the N‐terminal Cph2 fragment harbouring a red/far‐red interconverting phytochrome fused to a c‐di‐GMP degrading EAL domain restores the complex behaviour of the intact Cph2 photosensor. c‐di‐GMP was shown to regulate flagellar and pili‐based motility in several bacteria. Here we provide the first evidence that this universal bacterial second messenger is directly involved in the light‐dependent regulation of cyanobacterial phototaxis.</jats:p>